Material media change the properties of electromagnetic energy transmitted through them. Permittivity and permeability are electrical characteristics of material media. The permittivity of a medium describes how much electric flux is generated per unit charge in that medium. Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. Because material media have different electrical characteristics (i.e. different permittivity and permeability) with respect to electromagnetic energy, material media can be identified by their response to electromagnetic energy.